1. Field of the Invention
The present invention relates to friction material such as brake pads for automobiles and clutch facings, and more particularly, to a method of pelletizing friction material and a method of manufacturing preliminarily formed material for the friction material.
2. Description of the Related Art
The friction material is obtained by forming powdery material comprising fibrous material such as organic fibers, inorganic fibers, and metal fibers, friction adjusting agent including graphite and barium silicate, filler such as natural rubber and synthetic rubber, and thermosetting resin including phenol resin as a binder.
Hardening the powdery friction material under pressure and heat, and adhering the hardened material to a steel backing plate produce a disk brake pad.
FIG. 2 shows a backing plate used for a conventional disk brake pad. Striking a plate with predetermined shape from steel plate for automobiles or structural steel plate for machinery through press, and drilling two binder holes 2, 2, obtain the backing plate 1 shown in the figure.
FIGS. 3(a) to 3(c) show the construction of the press for adhering the friction material to the backing plate 1. The press is provided with an upper die 5, a frame 6, and a plunger 7. The frame 6 is fixed, and the upper die 5 and the plunger 7 are vertically movable. The upper die 5 is provided with two projections 8 at positions corresponding to the binder holes 2 of the backing plate 1.
To adhere the friction material to the backing plate 1, generally, two methods are applicable. In the first method, the backing plate 1 is directly piled upon the powdery material a, and the both are pressurized and heated to combine them with each other. To be concrete, as illustrated in FIG. 3(a), the plunger 7 is maintained at a predetermined height, and the powdery material a is fed to the frame 6 and is cut by rubbing to be flat. Then, as shown in FIG. 3(b), the steel backing plate 1 is held on the frame 6, and as shown in FIG. 3(c), the upper die 5 is lowered, and simultaneously, the plunger 7 is lifted, and the backing plate 1 and the powdery material a are heated. Melting and hardening thermosetting resin contained in the friction material a allows the friction material A to be adhered to the backing plate 1. At this moment, the friction material A enters into the binder holes 2, 2 also to improve binding force. The projections 8 are formed to substantially uniformly expand the friction material A, which enters the binder hole 2, 2.
In the second method, the powdery friction material a is fed to a metal mold and is preliminarily formed therein under pressure without heat, and the preliminarily formed material is piled upon the backing plate 1 in another metal mold and is adhered thereto under pressure and heat.
FIG. 4 shows the preliminarily formed friction material, in which (a) is the plan view, and (b) is the front view. The preliminarily formed material 3 has the same shape as final products, but its density is low, and its thickness T is approximately double in comparison to that of the final product that is adhered to the backing plate 1 under pressure and is pressed to have predetermined density. Further, the preliminarily formed material 3 is provided with mounds 4, 4 at the positions corresponding to the binder holes 2. The mound 4 has a wide base portion, but the diameter of the top thereof is smaller than that of the binder hole 2, so that the mound 4 can easily enter in the binder hole 2.
The preliminarily formed material 3 is fed to the frame 6 shown in FIG. 3, and the backing plate 1 is placed on the frame 6. Then, the upper die 5 is lowered, and the plunger 7 is lifted to adhere the friction material A to the backing plate 1 under pressure and heat. The mounds 4, 4 enter in the binder holes 2, 2 and are solidified therein to improve binding force between them.
The first method is advantageous in that the preliminarily forming process can be omitted. But, the method is not suitable for multiple productions. The “multiple productions” mean a method of preparing plurality of backing plates; placing a frame on each of the backing plates; feeding friction material in the frame; and adhering the friction material to the backing plate under pressure and heat to produce plurality of disk brakes at a time. Therefore, in the multiple productions of the first method, plurality of feeding devices are necessary since the friction materials are fed to plurality of metal molds. In addition, after the materials are fed, it takes time to cut by rubbing, for surface flatness, which causes the metal molds to be cooled.
On the other hand, in the second method, it is possible to store the preliminarily formed material produced, and the process of cutting by rubbing by the feeding device in the first method is unnecessary. Further, many disk brake pads can be produced at one time, resulting in improved working efficiency.
However, in the above methods, the powdery material contains sticky material such as NBR that is not vulcanized, so that such material sticks on the wall of the metal mold at the preliminarily forming process, which causes the preliminarily formed material to easily be broken when removed from the metal mold. This phenomenon may occur because sticky material contained in the friction material becomes finer than the surface of the metal mold, and enters concave portions on the surface of the metal mold, so that the sticky material expands or extends through pressing.
Meanwhile, disk brake pads are required to have stable braking capacity in wide temperature range since brakes with high performance have been used as the performance of vehicles have progressed. In this connection, friction material with base material of metal fibers with high heat-conductivity has high heat-conductivity itself, so that the heat generated on its friction face easily reaches the face contacting the plate through the friction material. As a result, the heat reduces the binding force between the friction material and the plate.
To solve the above problem, material laying stress on frictional efficiency has been used on the friction face side, and material laying stress on adhering performance to a plate and adiabatic performance has been used on the contact face. That is, friction material has been doubled or multiplied. One of such multiplied disk brake pads is disclosed in Japan Patent No. 3,409,426. In this patent, a friction material is divided into layers of pelletized material and material that is not pelletized, or one of the layers on the backing plate side after formation with pressure and heat is reduced in porosity in comparison to a layer on the friction side.
Mixing several materials, with different specific gravity, such as fibrous materials, friction adjusting materials, fillers, and thermosetting resins, as described above, produces powdery friction material. Therefore, when the friction material is left as it is, material with large specific gravity sinks and material with small specific gravity floats, that is, the friction material easily segregates, which causes its quality to vary widely.
In order to prevent such segregation, it is known that powdery material is firstly pelletized to tablets. In other words, powdery material is pelletized to small grains before it does not segregate. One of the pelletizing methods is disclosed in Japanese Patent Application Laid-open No. Showa 54-138053. In this document, asbestos and phenol resin powder are mixed; solvent that does not excessively dissolve the phenol resin is added to give moisture; and the mixture is pressed out of a pressure hole and is cut to produce pellets. With these pellets, the segregation can be prevented, and friction material with uniform quality can be obtained.
In another method of pelletization disclosed in Japan Patent No. 2,993,362, powdery material is heated at a temperature above the melting point of thermosetting resin and below the hardening temperature thereof; and the material is pressed to be discharged from a pressure hole; and the discharged material is cut to desired length to pelletize the material.
However, with the both methods disclosed in the second and third documents, pelletized material can not be preliminarily formed, because even through pressure is applied to the pelletized material desired shape is not to be maintained due to weak binding force.
In order to maintain the shape of the preliminarily formed material, it is necessary that fibers contained in the friction material should be interwound with each other. However, with the above methods, the fibers are coated with resin and rubber, which prevents the interwindings between the fibers.